CM
In the literature: contested
Pairing symmetry of heavily electron-doped FeSe without hole pockets
In plain words
In some FeSe compounds only electron pockets remain at the Fermi level, so the standard sign-changing scenario between hole and electron pockets cannot apply.
Precise statement
For monolayer FeSe on SrTiO3, (Li,Fe)OHFeSe and KxFe2-ySe2, which lack hole pockets at $\Gamma$, determine whether the gap changes sign between the electron pockets at $M$ (d-wave or bonding-antibonding $s\pm$) or is sign-preserving ($s++$). An answer is a phase-sensitive test, such as quasiparticle interference with magnetic and nonmagnetic defects or defect-bound in-gap states, with unambiguous sign assignment.
What would settle it
Phase-referenced quasiparticle interference or impurity-state spectroscopy on two such compounds yielding the same sign structure.
Related problems
- More general than Pairing symmetry of monolayer FeSe on SrTiO3